Until recently, the drying of fluids, either gases or liquids, by the selective adsorption of the water constituent was economically feasible only when the concentration of water in the fluid was small, i.e., present at a level of a few parts per million up to about 2.5 weight percent. For higher water-content feedstocks which were normally in the vapor phase, refrigeration processes were usually resorted to. In the case of high water-content normally liquid feedstocks, distillation procedures were most commonly employed. A principal difficulty encountered in attempted vapor phase processes was the phenomenon known as "crossover" in which the heat of adsorption of the water vapor within the adsorbent bed creates a heat front which remains within or behind the water mass transfer front as the two fronts advance along the bed. This resulted in a reduced efficiency of the adsorbent for water removal by the lowering of the effective water equilibrium capacity and the elongation of the mass-transfer front with ensuing early breakthrough of water into the product stream. In liquid phase processes, relatively short adsorption-regeneration cycles were mandated by the rapid exhaustion of the water-adsorption capacity of a fresh adsorption bed due to the high concentrations of water present in the feedstock. It was found, however, that it was extremely difficult to transport the high molar density liquid within the adsorption system during the regeneration and bed-filling stages of the cycle quickly enough to meet the demands of the imposed short cycle. In both types of processes, a major obstacle to commercialization was the lack of an economical bed regeneration procedure.
In U.S. Pat. No. 4,373,935 issued Feb. 15, 1983 to Ausikaitis et al. a method for successfully operating a vapor-phase bulk drying process for treating high water content feedstocks is described. In this process the inefficiency inherent in the unavoidable non-isothermal adsorption step caused by the crossover phenomenon is found to be overbalanced by utilizing the heat energy generated during adsorption to create a more efficient thermal regeneration step.
In the case of liquid phase operation, however, no significant breakthrough in devising a suitably economical bulk-drying process has been proposed in the prior art.